This page has the parts list and assembly instructions for the EKG-blinky board (rev 1.0, 2012 Aug 17). I am not planning to market the board (too much hassle, and I’d have to pay for a commercial license for the Eagle PC board layout software), but I thought I’d try writing up instructions as if it were a product. It takes under an hour to solder everything together, but most of a day to take photographs and write up the instructions.

I have not yet written up a description of the Arduino software for recording a long EKG signal, but there will be a page for that within a few months (my son is writing the software, and I don’t think he has finished documenting it yet).

Disclaimer: The EKG-blinky circuit is a hobbyist toy, not a medical-grade EKG! Do not use it for medical diagnosis. It’s great for geeky jewelry, though!

2-pin male header (for connecting scope probe or Arduino) Note: female header could be used instead, which might make cabling to an Arduino easier, but would increase the difficulty of attaching a scope probe.

3 alligator clips

3 different color wires (about 2′ long) twisted together

Electrodes

Disposable Ag/AgCl EKG electrodes with 4mm snaps

Assembly instructions

Start by bending each resistor’s leads 90°, with the ends 0.4″ apart. I found it convenient to use a “lead-forming tool” to get neat consistent bends, but a pair of pliers will work fine.

Slide the resistors into place from the component side, so that they lie flat on the board. Bend the leads slightly on the solder side, to hold the resistors in place.

Solder each resistor in place from the soldering side of the board and clip off the excess wire.

Place the two integrated circuit amplifier chips on the board, making sure that the notch on the chip package lines up with the marking on the board (so that pin 1 is close to the edge of the board). Be sure not to get the INA126P instrumentation amp and MCP6002 dual op amp chips mixed up! The leads may have to be gently straightened to line up with the holes on the board. Solder both chips in place from the soldering side.

Place the 4 4.7µF capacitors (but not the 0.47µF capacitor yet, as it may not be needed—it is the one that would go on the left side of the board just above the tab for the battery holder). Solder the capacitors in place and trim any excess wire.

Place the LED so that the flat side (and shorter lead) is toward the right edge of the board and the longer lead goes through the + hole. Solder the LED in place and trim the excess wire.

Solder the 3 terminal block in place so that the holes for the wires face out from the board.

Adjust the trimpot to the middle of its range (it is helpful to have an ohmmeter for this) and solder it in place, trimming off the excess wire.

Solder the pair of header pins in place.

Solder the battery clip in place, making sure that the three legs rest firmly against the PC board. If they don’t, then reheat the solder joints while pressing the clip firmly into place.

Inspect all solder joints to make sure that they are clean and shiny, and that they don’t bridge to adjacent wires.

Attach the alligator clips to one end of the wires and attach the other end of the wires to the screw terminals. The terminals are in order from the top of the board: ground, V-, V+.

The EKG-blinky is done!

The finished EKG blinky board!

To use the EKG blinky

Placement for electrodes

First, you need to attach EKG electrodes to your body. There are many different placements that can be used, but one fairly reliable one is to put the positive lead on the left shoulder (above the pectoral muscle) and the negative lead on the right rib cage (below the pectoral muscle), so that the line between the electrodes goes through the heart. The ground electrode can be placed almost anywhere on the body—I found it convenient to place it below the rib cage on the left side.

Before attaching the electrodes, it helps if the skin is clean and any dead skin is removed. Although one can use alcohol or acetone wipes for cleaning off dead skin, a simpler technique is to peel off a layer of dead skin with Scotch tape before sticking on the electrode.

Press the electrodes firmly to your skin so that the sponge with the electrolyte gel is compressed and the adhesive seals all the way around to keep it from drying up. If properly attached, the electrodes should work for a day, though you may find them a bit irritating well before then.

[UPDATE 2012 Aug 20. I don’t think that this placement of the electrodes is optimal. I’m going to investigate some other electrode placements to see if I can get signals that are more easily compared with published EKG signals.]

Put two CR2032 batteries in the battery clip. You can use other 20mm 3V batteries, like the CR2016 or CR2025, but they won’t last as long. The numbering scheme refers to the diameter (20mm) and thickness (3.2mm). Make sure that the + side of the batteries is up. The spring of the clip is conveniently labeled + to remind you.

Attach the alligator clips to the electrodes.

If you are lucky the LED will start blinking to your heart beat (on each R pulse). If not, you can try adjusting the gain of the second stage by turning the screw on the trimpot. Turning it clockwise decreases the gain, turning it counterclockwise increases the gain. If you are seeing a double flash on each pulse, then you have the gain set much too high, as the LED is lighting for the much smaller T pulse as well as the R pulse.

Lifting your left arm or tensing the left pectoral muscle should make the LED flicker, as the EMG signal from the left pectoral muscle swamps out the EKG signal from the heart.

If you have an oscilloscope, you can attach it to the header pins to observe the wave form. If you see a big downward spike on each pulse, you have the plus and minus leads reversed. If you see a lot of 60Hz (50Hz in Europe) hum, then your ground wire is not connecting properly, or you forgot to twist the wires together to reduce inductive pickup of magnetic fields. If there is a lot of high-frequency noise, you can add the 0.47µF capacitor to the board just below the screw terminals—it was not necessary for me.

If you have an Arduino, you can connect the header pins to GND and one of the analog input pins and record the voltage every 5 msec. Plotting that waveform should give a very clean EKG signal. You can examine the plot to see if the gain is set too high.

Trace showing gain set properly for viewing the waveform. The top of the R pulse goes to almost 1000, but does not exceed it. Note that a real EKG would have a calibrated gain, not a continuously adjustable one, so that the output could be interpreted as specific voltages at the electrodes. I had to turn the gain way down from the initial middle of the potentiometer setting before getting it right. [Click image for larger view]

This plot shows what the trace looks like when the gain is set too high. The maximum value (1023) is seen for several samples in a row, indicating that clipping of the R pulse has occurred. Note that the LED will still flash quite nicely at this gain, but the analog output goes too high for the Arduino to record (and may be saturating the output of the op amp also). [click on image for larger view]

To turn the EKG blinky off, either remove the batteries, or slip a piece of thin cardboard between the spring of the battery clip and the batteries, to interrupt the circuit.

Update (2012 Aug 20): Better electrode placement.

After talking with an EKG expert, I changed the placement of the electrodes to correspond more to the conventional limb electrodes (LA=left arm, RA=right arm, LL=left leg). I didn’t want to use limb electrodes, because there are then more muscles that can cause EMG interference, so I put the electrodes on the torso about where the limbs connect.

Connection of electrodes to get “Lead I” which is LA-RA. The top two electrodes should probably be placed more symmetrically—I think that the right-arm (–) electrode may be better placed than the left-arm (+) electrode.

A portion of the Lead I signal. The PQRST are all quite clear. The voltage scale is approximate, based on a computed gain for the EKG blinky amplifier. The zero-line is set by averaging the readings for about 10 seconds of recording. The sampling period was 3 msec.

The Lead II electrodes (LL-RA). This is the conformation that the EKG expert said would most likely give the canonical waveform for a normal, healthy individual.

The waveform for Lead II is indeed quite strong, with very clear P and T waves. Note: despite the same time scale, the Lead I, Lead II, and Lead III recordings were all done separately, with the wires moved between recordings. They do not correspond to the same heartbeats.

The arrangement of wires for Lead III (LL-LA).

The R wave is very hard to see in Lead III, probably because the dipole is oriented almost horizontally, so the vertical lead placement sees little difference in the field.

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